Austenitic alloys



Aug. 28, 1962 mam/m Noaoa R. K. PITLER ETAL 3,051,565

AUSTENITIC ALLOYS Filed April 29, 1960 RUPTURE LIFE, HRS

. O O I i I\ 3 i 5 '0 J O U I I l I IO- uvmvrons Edward E. Reynolds Richard K. Piiler George Aggen BY ATTORN United States Patent 3,051,565 AUSTENITIC ALLOYS Richard K. Pitler, Oakmont, Edward E. Reynolds, Allison Park, and George Aggen, Sarver, Pa., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Filed Apr. 29, 1960, Ser. No. 25,790 4 Claims. (Cl. 75134) This invention relates to austenitic alloys and in particular to solution hardened and precipitation hardening austenitic intermediate iron-nickel base alloys containing chromium, tungsten, molybdenum, titanium, aluminum and boron as major elements, said alloys being suitable for use at elevated temperatures and in particular at temperatures of up to at least 1500 F.

Engineers of today are designing power units and structures for use especially in supersonic aircraft and guided missiles which require the units to operate at higher and higher temperatures for increased power and efficiency. With the ever increasing temperature of op eration of modern day power units new alloys must be developed which are suitable for use at higher temperatures of operation. In particular, for such applications as turbine wheels, turbine buckets and stationary parts which are designed for use at temperatures of up to at least 1500" F. and higher the alloy from which such components are formed must be suitable for use at such elevated temperatures possessing the optimum combination of high strength, excellent ductility and resistance to corrosion and oxidation at the elevated temperatures encountered in operation. At the same time these alloys must be capable of easy fabrication from sheet, bar, cast or extruded forms of semi-finished mill products, be relatively cheap and contain a minimum amount of costly and strategic alloying elements.

Heretofore most of the iron and iron-nickel base alloys suitable for uses as described hereinbefore have been limited to an operating temperature of not over 1300 F. These alloys have been efiective for use in these operations, that is, they have sufiicient strength, ductility and corrosion resistance to adequately meet the demands of a power unit operating at temperatures of not over 1300 F. Where, however, the temperature of the power unit or the structure is designed for operation at a temperature in excess of 1300 F. and, in particular, at temperatures of up to at least 1500 F. and higher recourse has almost been universally had to nickel base alloys or cobalt base alloys. When such parts are produced from the nickel or cobalt base alloy they are extremely costly, are difiicult to fabricate and utilize a great quantity of strategic alloying elements. Thus, there is an apparent need for an intermediate iron-nickel base alloy to replace the nickel or cobalt base alloys, the intermediate iron-nickel base alloy being characterized by using a minimum of strategic alloying elements and which can be easily fabricated and is relatively inexpensive yet possesses mechanical properties comparable to those possessed by solely nickel or cobalt.

In addition, the prior art has also taught that in predominantly nickel base alloy, that is, an alloy containing between 30% and 80% nickel, the iron content must necessarily be limited. An arbitrary figure of 20% iron has been established as the upper limit above which a nickel-iron base alloy would possess highly inferior mechanical properties. Preferably the iron content is always maintained at less than 8% in order to provide the alloy with its optimum combination of mechanical properties.

An object of this invention is to provide a solution hardened and precipitation hardening intermediate ironnickel base alloy having a minimum amount of costly and strategic alloying elements, said alloy being suitable for use at temperatures of up to at least 1500" F. and higher.

Another object of this invention is to provide a solution hardened and precipitation hardening austenitic intermediate iron-nickel base alloy having optimum combination of strength, ductility, corrosion and oxidation resistance when used at temperatures of up to at least 1500 F. and higher.

A more specific object of this invention is to provide a solution hardened and precipitation hardening austenitic intermediate iron-nickel base alloy having greater than 20% iron contained therein and having as essential alloying elements chromium, aluminum, titanium, molybdenum, tungsten and boron and which is capable of being precipitation hardened by heat treatment and is suitable for use at temperatures of up to at least 1500 F. and higher.

Other objects of this invention will become apparent when taken in conjunction with the following description and the drawing, the single figure of which illustrates the effect of the boron content on the rupture life of the alloy of this invention.

In its broader aspects the alloy of this invention comprises up to about 0.12% maximum carbon, up to about 1.5% maximum manganese, up to about 1.5% maximum silicon, from about 13.0% to about 16.0% chromium, from about 42.0% to about 48.0% nickel, from about 0.7% to about 2.0% aluminum, from about 2.0% to about 3.75% titanium, the sum of the titanium and aluminum being within the range between about 3.0% and about 5.75%, from about 3.0% to about 5.0% molybdenum, from about 2.0% to about 5.0% tungsten, the sum of the molybdenum and tungsten being within the range between about 6.25% and about 9.0% up to about 1.0% vanadium, from about 0.007% to less than about 0.03% boron, preferably not more than 0.025%, and from about 25.0% to about 32.0% iron with not more than about 1.5% of incidental impurities, such as nitrogen, phosphorus, sulfur and the like. Each of the essential alloying elements performs a specific function within the alloy of this invention.

The carbon is present as an impurity and must be limited to about 0.12% maximum in order to prevent the formation of deleterious carbides in great proportions, especially titanium carbide which, if formed, precipitates as an unwanted constituent and adversely aifects the strength and ductility of the alloy. In addition, carbon contents in excess of 0.12% produce excessive amounts of titanium carbide thus decreasing the amount of titanium available for precipitation hardening thereby seriously decreasing the strength of the alloy. Manganese and silicon are used for purposes of purifying the alloy but do not substantially contribute otherwise to the strength, ductility or corrosion resistance. Chromium, on the other hand, is necessary to provide the alloy with adequate resistance to corrosion and oxidation, especially when used at temperatures of up to at least 1500 F. and higher. At least 13.0% chromium is required in order to afford the alloy with an adequate resistance to corrosion and oxidation whereas chromium contents in excess of 16.0% adversely affect the strength of the alloy. Optimum combination of properties appears to be obtained when the chromium content is maintained within the range between about 14.0% and about 16.0% in that the alloy is afforded suflicient corrosion and oxidation resistance without adversely affecting the strength and in particular the ductility of the alloy.

While nickel is an essential element in the alloy of this invention and at least 30% is required in order to obtain the optimum conditions for precipitation hardening the alloy, the present alloy contains a minimum of 42.0%. The nickel content should be limited to 48.0% maximum since nickel contents in excess of 48.0% do not appear to be of any further benefit in effecting precipitation hardening or otherwise enhancing the properties of the alloy. In addition, nickel contents in excess of 48% contribute considerably to the cost of-producing the alloy. Optimum combination of properties appears to be obtained where the nickel content is maintained within this range. This optimum range appears to give the optimum balance between strength and ductility without inducing difiiculties in fabrication or without excessively increasing the cost of the alloy.

Aluminum and titanium are the elements which cooperate with nickel to form a precipitation hardening alloy. It appears that within the general range of elements set forth hereinafter in Table I the aluminum and titanium cooperate with the nickel to form a precipitation hardening constituent, possibly Ni (Al, Ti), and each may be partially substituted one for the other. It has been found, however, that in order to obtain the optimum strength characteristics of this alloy both aluminum and titanium must be present. It has been found that a minimum of 0.7% aluminum and about 2.0% titanium is necessary for proper strengthening of the alloy through precipitation hardening, yet the sum of the aluminum and the titanium contents must not be less than 3.0%. In other words, while at least 0.7% aluminum and at least 2.0% titanium must be present, the minimum amount of aluminum and titanium must be at least 3.0%. Aluminum contents in excess of about 2.0% are not beneficial as they do not further strengthen the alloy and have a deleterious effect upon its hot workability. While both aluminum and titanium are essential, the sum of the aluminum and titanium contents should not exceed 5.75% because of the danger of precipitating deleterious phases of a highly complex nature which adversely affect hot workability and cause reduced elevated temperature ductility. Titanium contents in excess of about 3.75% appear to cause conditions which are favorable to the precipitation of a deleterious phase embodying titanium and thus lessens the amounts of titanium available for the formation of the precipitation hardening constituent with nickel and aluminum. Optimum strength and ductility appear to be imparted to the alloy of this invention when the sum of the aluminum and titanium contents is within the range between 3.4% and 4.6% with at least 0.7% of alumi num and at least 2.0% of titanium present therein.

When thealloy of this invention is made by the Wellknown air melting procedure, it is found that vanadium in an amount of up to 1.5% is necessary in ofisetting the embrittling tendency of the titanium through its union with oxygen in the resulting alloy. However, where the alloy is produced by a vacuum melting procedure, vanadium may be present only in traces.

As was stated hereinbefore, the alloy of this invention is solution hardened and precipitation hardening. It thus becomes apparent that not all of the hardening or strengthening is due to precipitation hardening of the alloy. The molybdenum and tungsten contents as set forth hereinafter in Table I are effective for strengthening the solid solution of the alloy. In this respect, it has been found that a molybdenum content of at least 3.0% is needed to show any effective strengthening of the solid solution. Molybdenum contents in excess of 5 .0% do not appear to increase the strength to any substantial degree and may be eifective for causing the precipitation of embrittling phases and contribute to the phenomenon of catastrophic oxidation. A tungsten content of at least 2% is needed for solid solution strengthening, whereas tungsten contents in excess of 5.0% do not appear to increase the strength to any marked degree. In addition, excessive tungsten is highly costly and strategic. 'In this respect, it must also be noted that the tungsten content cannot be substituted for the molybdenum content and vice versa. Both of the elements are necessary and the elimination of one with a corresponding increase in the other will not produce the optimum combination of mechanical properties capable of being exhibited by the alloy of this invention. It is preferred to have the sum of tungsten and molybdenum contents within the range between about 6.25% and about 9.0% and with a minimum of at least 3.0% molybdenum and at least 2.0% tungsten. In other words, while the alloy may contain as little as 3.0% molybdenum or as little as 2.0% tungsten, yet the combined molybdenum and tungsten contents must be within the range between 6.25% and 9.0%.

Of particular significance is the boron content of the alloy of this invention. It has been found that with a minimum of about 0.007% boron a great increase in strength is noted over the alloy without boron. This great increase in strength is accomplished without any adverse efiect upon the ductility of the alloy when used at temperatures of up to 15 00 F. Increasing the boron content 0.03% or above greatly affects both the strength and duetility to such a degree that the alloy is not commercially usable at temperatures of up to 1500= F. It is preferred, however, to maintain the boron content at 0.025% maximum. The optimum combination of mechanical properties appears to be obtained when the boron content is maintained within the range between 0.007% and 0.016%. Boron also functions to permit the use of higher amounts of titanium and aluminum than heretofore possible in alloys devoid of boron without the formation of complex phases which affect the strength and ductility of the alloy. In addition, the use of boron and the associated higher amounts of titanium and aluminum is effective for increasing the mechanical properties of the alloy and also imparts excellent forgeability characteristics to the alloy which heretofore was forgeable only to a very limited degree andin some cases, not at all. The balance of the alloy is iron which must be present within the range between about 25.0% and about 32.0%. Optimum combinations of mechanical properties are produced when the iron content is within the range between about 25 0% and 32.0%. Not more than 1.5% of incidental impurities of the type usually found in such alloys, for example, nitrogen, phosphorus and sulfur can be tolerated.

Reference may be had to Table I illustrating the general range and the optimum range for the chemical com.- position of the alloy of this invention, it being noted that these ranges also include the incidental impurities in the amounts as set forth hereinbefore.

Table I.Chemzcal Composztzon (Wt. percent) Element General Optimum Range Range o 0.12 max. 0.10 max.

Mn 1.5 max. 0.5 max. 81 1.5 max 0.5 max. Cr 13.0 16 0 14.0-16.0 N1 42.0-48 0 42.0-48.0 Mn 3.05.0 325-45 W 2.05.0 3.0-4.5 7 M o|-W 625- T1 2.0-3.75 2.7-3.3 l 0.7-2 0 0.71.3 3.44.6 l 0.50 max.

Percent RA.

Hours to Per- Rupture Stress (p.s.i.)

00000000000 0000000000000000 000000000000000000000000000 000000000000000000000000000 fi 0 0 fiw0 0 00 5 5 5 :w5 5 :w.o nvxwewzwro fiwswewxm 888888888222222222222222222 lnvention is clearly illustrated.

Properties Test Percent B Temp.

When the alloys are tested at 1200 -F.

L0 0 LL1 .0 LLLLLLZ2&Z

to Section A of Table III the effect of Table III.--Efiect of B Additions 0n Stress-Rupture 1550 F.24 HRS.-AIR COOL 1400 F.16 HRS.AIR COOL Heat The alloys of this invention may be made in any of the Well-known steel mill manners, for example, carbon electrode electric arc furnace melting, induction furnace HEAT TREATMENT; 205QF -1 melting, vacuum melting and consumable electrode vacuum melting. In any event the melt, after refining to 5 the proper chemical analysis, is usually cast into ingots. The ingots are thereafter hot formed to the desired semifinished mill product by forging, pressing, rolling, extruding or in any other convenient manner.

The alloy iisiiriIIIIIIIII in this form can be solution heat treated, to be referred to more fully hereinafter, and thereafter quenched from the solution heat treatment temperature thereby softening the alloy to permit the alloy to be easily fabricated into H 175 the desired component. These components can be formed from any of the usual semi-finished mill products, for example, sheet, strip, bar or extruded shape. 2V003A Thereafter the fabricated part is subjected to a precipitation hardening heat treatment, to be more fully described hereinafter, to impart to the finished article the optimum 20 combination of strength, hardness and ductility within the alloy or the optimum single mechanical property as desired. The alloy may also be used as a casting alloy for forming cast articles of predetermined shape, the mechanical properties being developed or imparted thererring 6169889 57180660504. 000005 500060035 MHM 3219%875547m1447 5fi45% Refe creasing amounts of boron on the stress rupture properties of the alloy of this The alloys of Section A have been :given the double hardening heat treatment according to the schedule set forth therein.

and under a stress of 80,000 p.s.i., it is seen as illus- 4 A AI A A .L .5 4.3334444 33 4 111 1333355 56777 taaatttttaateasta 219 90887827t0h82 1357 L45 81357 1 6 157 4444 111MM111luuMMMMMM5444455544554465 5654 11111111111111 111 Table II.Chemical Analyses In order to show the eifect of the alloying elements and the effect of the heat treatment on the alloys f this invention, reference may be had to Table II which he cal composition both Within and outside of the general range as set forth hereinbefore in Table I.

Heat

to by the precipitation hardening heat treatment to be described here after.

contains the chemical analysis of a number of alloys having a c Reference may be had to Table III illustrating the effect trated by Heat Nos. J-ll, :H-520, 6X-614 and H-l53 of boron on the stress reputre properties of the alloy that increasing the boron content from 0.0005% to It is to be noted that in cer ain 7 0.003% has little effect upon the rupture life or the g applications as, for example, where the alloy rupture ductility of these alloys as measured by the peris to be subjected to high stresses at temperatures of cent elongation and percent reduction of area. However, when the boron content is increased to within the range between 0.007% and 0.03% as illustrated by Heats 2V-003A, 2V-003B, 2V-003C, and 2V-003D, it

of this invention.

gineerin up to at least 1500 F. and higher, the stress rupture properties are the principal criteria for evaluating the properties of the alloy for that purpose.

is seen that a great increase in the rupture life is noted without any adverse eifect upon the ductility of the alloy. In fact, when the ductilities of Heats J-ll and 6X-614 are compared with the ductilities for Heats 2V003A, 2V-003B, 2V-003C and 2V-003D, it is apparent that the ductility of these latter alloys is quite favorable. Where, however, the boron content is increased to beyond 0.03% as illustrated by Heat H175, it is seen that while a high ductility is available the rupture life has been greatly decreased.

Substantially similar results are noted when the same alloys were tested at 1500" F. and under a stress of 25,000 p.s.i. Here boron contents below 0.007% show vastly inferior rupture life and ductility, whereas the alloys having a boron content within the range bet-ween 0.007% and 0.03% not only show a markedly superior rupture life but an excellent ductility when compared with alloys having a lesser amount of boron. Increased amounts of boron beyond the upper limit of 0.03% once again have the effect of decreasing the rupture life as well as the ductility.

Section B of Table HI illustrates the etfect of the boron on the stress rupture properties of the same alloys which were also tested at 1500 F. and at a stress of 25,000 p.s.i. It is to be noted that the only'ditference between the alloys of Section B and those tested at the same temperature and stress in Section A is the difierence.

in the heat treatment. As effectively illustrated in Section B, the heat treatment of these alloys is efiective for producing the same results but of a ditferent degree. These heat treatments will be described in more detail hereinafter. As illustrated by the test results recorded in Section B, boron contents of less than .007% produce poor rupture life and in particular poor ductility. Where, however, the boron content is increased within the range given hereinbefore in Table I and in particular in the range between 0.007% and 0.03% as illustrated by alloys 2V-003A, 2V-003B, 2V-003C and 2V-003D, it is seen that the alloys of this invention possess an excellent rupture life and an excellent ductility. Increasing the boron content up to 0.244% is effective for decreasing the rupture life and the ductility of these alloys.

An outstanding feature of this invention is clearly illustrated by reference to Table IV. Table IV illustrates the efiect of boron on the transverse and longitudinal stress rupture characteristics of this alloy under various states of stress and temperature.

Table 1V.Efiect of Boron on Transverse Properties HEAT TREAIh/IENT: 2050 F1 LIL-OIL QUENGH+1550 F 24 Hrs.AIR COOL-f-l400 F.16 Hrs.AIR COOL [A. Stress rupture properties] per- Hours Per- Per- Heat cent Direction Temp. Stress to cent cent B F.) (p.s.i.) Rup- El ed ture 3A .007 Lon 1,200 90,000 116 8.0 12.2 2V4) T1655"- 1, 200 90, 000' 119 12.0 15.9 Long '1, 200 90, 000 100 9. 1 14. 3 Traus 1, 200 90,000 126 12.0 16. 6 Long 1, 200 80, 000' 198 3. 3 6. 2 Traus 1, 200 80, 000 138 5. 2 10. 8 1, 200 100, 000 18 15. 2 16. G 1, 200 100, 000 14 11. 1 18. 9 1, 200 90, 000 126 8. 4 14. 5 l, 200 90, 000 102 12. 15. 2 1, 200 90, 000 119 10, 0 l3. 3 1,200 90,000 116 7. 4 12. 4 1, 500 35, 000 150 16. 33. 2 500 35, 000 143 16. 0 25. 7 1, 500 35, 000 140 19. 8 33. 2 1, 500 35, 000 106 14. 0 22.0 1, 500 25,000 568 12. 1 29. 5 1, 500 25, 000 522 11. 4 24. 8 1, 500 40, 000 44 9. 6 17. 4 1, 500 40, 000 36 7. 9 18. 5 1, 500 35, 000 123 28. 8 36. 4 1, 500 35, 000 117 12. 0 2t. 0 1, 500 35, 000 113 15.3 36. 3 2V-003D .030 1, 500 35, 000 142 18.0 27. 8

' known and used alloys.

As clearly illustrated in Table IV, there is an extremely close comparison of both the transverse and longitudinal rupture properties as produced by the alloy of this invention. master rupture curve using the Larsen-Miller parameter, a very close correlation is noted. While the efiect of boron is clearly illustrated in Tables III and IV the par-.

ticular role that it plays in producing the mechanical properties exhibited by the alloy is unknown. Photomicrographs of the alloys clearly illustrate that increasing amounts of boron stringers distributed in the longitudinal direction are prevalent with increasing boron contents within the range between 0.007% and 0.03%. These boron stringers are not visible at a magnification of x where the boron content is below about 0.007%. Heretofore the appearance of such stringers was thought to be etfective for adversely affecting the mechanical properties of the alloys. Such is not the case, however, with the alloys of the present invention as clearly substantiated by the test results recorded hereinbefore. However, increasing the boron content above the upper limit as given hereinbefore in Table I produces increasing amounts of the boron phase stringers which produce an adverse effect on the transverse mechanical properties in the resulting alloy.

An associated phenomenon is also noticed with the use of boron in the alloy of this invention. Heretofore the sum of the aluminum and titanium additions to an alloy without boron was necessarily limited becaus of the difficulties encountered in forging the alloy. With the use of boron, however, this problem is eliminated. Reference is directed to Table V which illustrates the effect of varying amounts of titanium and aluminum on the tensile properties of the alloy of this invention having about 0.008% boron contained therein.

Table V perper- 02% 4D per- Heat cent cent Y.S. 2% Y.S T.S. percent cent Ti Al 7 El RA 3. 1.17 81,000 98, 500 113,500 10.0 17. 8 3. 1. 09 92, 100 107, 900 122, 200 16. 8 30. 5 4. 1.03 84, 700 107, 500 124,000 15. 2 28. 0 1.78 2. 53 78,800 95, 400 112,100 15.4 31. 4 2.03 2.63 64, 800 92, 500 114, 500 14. 6 31.4 BV-223-4. 2. 55 2. 68 62, 300 97, 600 127, 000 14. 6 36.8 BV-222-4. 1. 64 3.15 51, 600 81, 500 104,000 19. 2 57. 6

As clearly illustrated for Heats 7M-102, BV-218, BV-219, BV-220, BV-22l, BV-222 and BV-223 it is seen that with a combined titanium and aluminum con tent within the range between 4.31% and 5.23% excellent ductility is obtained when these alloys are tested at 1400 'F. The elevated temperature tensile properties are excellent when compared with some of the-presently This appears to be the result of the combination of the boron content coupled with the higher titanium and aluminum contents. Moreover, excellent forgeability is imparted to the alloy in addition to the excellent mechanical properties exhibited by the test results recorded in Table V. It is desired to have titanium in the greater amount than aluminum but in any event it is necessary that at least 2.0% titanium be present and at least 0.7% aluminum being present Within the alloy, the sum of the aluminum and titanium being within the range between about 3.0% and 5.75%.

Reference is directed to Table VI which illustrates the effect of varying the tungsten and molybdenum contents on the rupture life of the alloys of this invention.

If these data are plotted on the well-known.

9 Table VI.Efiect of Molybdenum and Tungsten HEAT TREATIVIENI: 1,900" F.1 HR.WATER QUENOH+ 1,550 F.6 HRS.AIR COOLI-1,300 F.-16 HRS.AIR COOL Test Temp. 1,200 F. Test Temp. 1,500 F.-

Stress 94,000 p.s.i. Stress 30,000 p.s.i.

Heat N0.

Rupture Reduction Rupture Reduction Life (Hrs) of Area Life (Hrs) of Area (Percent) (Percent) 1. 3 3. 0 9. 5 6. 6 13. 0 14. 8 47. 0 19. 6 16. 0 8. 5 165. 0 23. 8 50. 0 12. 0 120. O 42. 6 33. 0 21. 8 44. 0 59. 7 23. 0 5. 3 136. 0 22. 7 50. 0 l0. 0 178. 0 35. 1 47. 0 16. 3 107. 0 39. 5 64. 0 19. 9 62. 0 58. 7 l9. 0 4. 3 114. 0 20. 7 43. 0 6. 0 147. 0 34. 7 44. 0 9. 9 59. 0 41. 5 63. 0 5. 0 46. 0 44. 2

From the test results recorded in Table VI and in particular for alloys I-a, I-b, Ic, I-d and Ie, it will be noted that when the alloys are devoided of tungsten and the molybdenum content is varied between nominally 0% and 8% the alloys having a balance nominal analysis of about 0.05% carbon, 0.35% manganese, 0.4% silicon, about chromium, about 44% nickel, about 3.4% titanium, about 1.0% aluminum, about 0.01% boron and the balance iron, increasing the molybdenum content is efiective for increasing the rupture life both at 1200 F. and 1500 F. and at stresses of 94,000 p.s.i. and 30,000 p.s.i., respectively. It is to be noted, however, that when the alloys are devoid of tungsten, the alloys posesss substantially low ductility as measured by the percentage reduction of area. On the other hand, as illustrated by the test results recorded for alloys II-a, IIb, II-c, II-d and IIe which contain nominally about 2% tungsten and vary from 0% and 8% molybdenum, it is apparent that optimum results are obtained when the combined tungsten and molybdenum content is in the range between 6.25% and 9.0%. In particular, alloy II-d which contains nominally 2% tungsten and 6% molybdenum when tested at 1200 F. and at 94,000 p.s.i. has a rupture life of 50 hours which far surpasses alloy I-e which contains about 8% molybdenum and exhibits a rupture life of 7.0 hours. Substantially similar results are obtained when these same alloys are tested at 1500 F. and under a stress of 30,000 p.s.i. Alloys III-a, IIIb, III-c and 111-0! which contain nominally 4% tungsten and a varying molybdenum content of between 2% and 8% in approximately 2% increments were tested under the same conditions and these tests clearly illustrate that the alloy containing about 4% tungsten and about 4% molybdenum exhibits optimum rupture life both at 1200 F. and at 1500 F. and at stresses of 94,000 p.s.i. and 30,000 p.s.i. respectively. These alloys, that is, the alloys containing about 4% tungsten and about 4% molybdenum also possess excellent ductility. By increasing the tungsten content to about 6% and varying the molybdenum content between about 0% and 6%, again in 2% increments, as more clearly set forth in the test results recorded for alloys IVa, IV-b, IV-c and lV-d, it is again clear that a combined tungsten and molybdenum content of about 8% is effective for imparting optimum properties to this series of alloys. This is more clearly illustrated with respect to the data of the rupture life of 1500 F. and under a stress of 30,000 p.s.i. Alloys V-a, V-b, V-c and Vd illustrate the effect of increasing the molybdenum content from about 0% and about 8% on an alloy containing about 8% tungsten. It

10 will be noted that as the combined tungsten and molybdenum content exceeds about 9.0%, the rupture life is seriously impaired especially at temperatures of 1500 F. Note in particular that alloy Ie is somewhat better than alloy Va, each of which contains 0% tungsten and 8% molybdenum and 0% molybdenum and 8% tungsten, respectively. Yet, both of these beats are substantially inferior to alloy 1114) which contains about 4% tungsten and about 4% molybdenum. By comparing alloy II-d and alloy IVb which contain 2% tungsten and 6% molybdenum and 6% tungsten and 2% molybdenum, respectively, it would appear that these alloys are substantially interchangeable with respect to their tungsten and molybdenum contents; however, by comparing the aforementioned two heats with alloy III-b, it is again apparent that the optimum combination of properties is obtainable only where the molybdenum content is between 3% and 5% and the tungsten content is between 2% and 5% with the combined tungsten and molybdenum content being within the range between 6.25% and about 9.0%. It will be evident that a plot of the properties as tabulated at Table VI will clearly substantiate this. Tensile tests, both at room temperature and at 12,000 E, clearly corroborate the conclusions as drawn from the stress rupture tests.

As was stated hereinbefore, the alloy of this invention is a precipitation hardening alloy which is suitable for use at temperatures of up to at least 1500 F. and higher. However, it becomes necessary to design a heat treatment which is capable of imparting optimum properties to the alloy. For example, it has been found that two separate heat treatments may be employed to impart the desired mechanical properties to the alloy of this invention depending upon the temperature at which the article is intended to be used. It is preferred to heat treat the alloy of this invention by a two-stage precipitation hardening heat treatment where the alloy is intended to be used at a temperature of about 1200 F. On the other hand, if the intended temperature of operation is at 1500 F., high rupture strength can be obtained through the use of a single-stage heat treatment.

More particularly, the two-stage precipitation hardening heat treatment consists of solution heat treating the alloy at a temperature in the range between 1750 F. and 2150 F. and preferably between 2000 F. and 2100 F. for one-half to two hours followed by rapid quenching in Water, oil or air. The solution heat treatment is necessary to place all of the precipitation hardening elements in solution within the matrix of the alloy in order to obtain the full effect of these elements thereby obtaining the optimum balance of mechanical properties. The alloy in the solution heat treated form is soft and ductile and can be easily fabricated into the finished product. Thereafter, the alloy in the form of the finished product is subjected to the initial aging step which consists of an aging heat treatment at a temperature in the range from 1450 F. to 1650 F. and in particular at a temperature in the range between 1525 F. and 1600 F. for a time period of between /2 and 24 hours and preferably between 18 and 24 hours and thereafter air cooling. This is followed by a second aging heat treatment at a temperature in the range between 1200 F. and 1400 F. and in particular between 1325 F. and 1400 F. and for a time period of between /2 and 24 hours and preferably between 12 and 20 hours and thereafter air cooling. This two-stage precipitation hardening heat treatment is designed to obtain high ductilities in the alloy of this invention when the alloy in the form of the finished wrought or cast product is used as a temperature of up to about 1200 F. On the other hand, if the alloy is to be used at a temperature of about 1500 F., it has been found that a single-stage precipitation hardening heat treatment is effective for developing the optimum stress rupture properties. The single-stage heat treatment consists of a solution heat treatment at a temperature in the range between 1750 F. and 2150 F. and especially within the range between 2000 F. and 2100" F. for about one-half totwo hours followed by a rapid quench in water, oil or air. Thus, the same solution heat treatment temperatures and times are used in each instance. Thereafter, the alloy in the form of the finished wrought or cast product is precipitation hardened by heating the alloy at a temperature in the range between 1250 F. and 1450 F. and in particular between 1350 F. and 1425' F. for about /2 to 24' hours and preferably between 12 and 20 hours and air cooling. 7

In order to more clearly illustrate the effect of boron on the rupture life when the alloys are heat treated in each of the manners described hereinbefore, attention is directed to the curves of the figure. Curve 10 illusstrates the elfect of boron on the rupture life when the alloy is double aged according to the schedule as set forth hereinbefore and curve 12 illustrates the effect of boron on rupture life when the alloys of this invention are given the single precipitation hardening heat treatment. As clearly illustrated by curve 12 boron contents below about 0.007% are ineffective for producing adequate rupture life. Boron contents in excess of about 0.03% are eifective'for decreasing the rupture life at a rapidly increasing rate. Optimum rupture life appears to be obtained when the boron content is maintained within the range between about 0.007% and about 0.016%. While it would appear from curve 10 thatanadequate rupture life could be obtained in an alloy having less than 0.007% boron, it is submitted that the ductility of these alloys is exceedingly poor to such a degree that the alloy is commercially unsuitable. As

clearly illustrated, optimum results are obtained whenthe boron content of the alloy is maintained Within the.

range as set forth hereinbefore.

The alloy of this invention requires no special skills or knowledge in producing the alloy or in the heat treatment applied thereto. All operations can be performed on existing equipment by anyone skilled in the metal lurgical art. The alloy has excellent properties suitable for use at temperatures of up to at least 1500 F. and higher and uses a minimum amount of strategic alloying elements thereby rendering the alloy more readily available while substantially reducing the cost thereof in rela-- tion to presently known and used superalloys, yet the alloy has substantially similar properties to the presently known and used superalloys.

Throughout the foregoing description, reference has been made to an intermediate iron-nickel base alloy. By the use of this term, it is intended that the alloy of this invention comprise at least 50% of the sum of the iron and nickel contents, each of which is within the respective range as set forth hereinbefore.

This is a continuation-in-part of my co-pending application Serial No. 717,134, filed February 24, 1958, now abandoned.

We claim:

1. A precipitation hardening wrought intermediate ironnickel base alloy suitable for use at elevated temperatures of up to at least 1500 F. and higher, consisting of, less than 0.12% carbon, less than 1.5% manganese, less than 1.5% silicon, from 13.0% to 16.0% chromium, from 42.0% to 48.0% nickel, from 3.0% to 5.0 molybdenum and from 2.0% to 5.0% tungsten, the sum of the molybdenum and tungsten being within the range between 6.25% and 9.0%, from 2.0% to 3.75% titanium and from 0.7% to 2.0% aluminum, the sum of the aluminum and titanium being within the range between 3.0% and 5.75%, less than 1.0% vanadium, from 0.007% to less 12 than 0.03% boron, and from 25.0% to 32.0% iron with incidental impurities, said alloy being characterized by exhibiting substantially the same level of rupture properties when measured in the longitudinal and in the transverse directions.

2. A precipitation hardening wrought intermediate iron-nickel base alloy suitable for use at elevated temperatures of up to at least 1500 F. and higher, consisting of, 0.10% maximum carbon, 0.05% maximum manganese, 0.5% maximum silicon, from 14.0% to 16.0%-

chromium, from 42.0% to 48.0% nickeLVfrQm 3.25% to 4.5% molybdenum together with from 3.0% to 5.0% tungsten, the sum of the molybdenum and tungsten being, within the ran e between 6.25% and 9.0%, from 2.7% to 3.3% titanum, from 0.7% to 1.3%' 'aluninum, the sum of the aluminum plus titanium being'within the range between 3.4% and 4.6%, 0.5 maximum vanadium, from 0.007 to 0.016% boron, and from 25.0% to 32.0% iron with incidental impurities and which is characterized by exhibiting substantially the same level of rupture properties when measured in the longitudinal and in the transverse directions.

3. An article of manufacture for use in highly stressed parts operating at temperatures of up to at least 1500 F. and higher formed from an austenitic solutionhardened and precipitation hardening intermediate iron-nickel base alloy consisting essentially of, less than 0.12% carbon, less than 1.5% manganese, less than 1.5% silicon, from 13.0% to 16.0% chromium, from 42.0% to 48.0% nickel, from 3.0% to 5.0% molybdenum and from 2.0% to 5.0% tungsten, the sum of the molybdenum and tangsten being within the range between 6.25% and 9.0%, from 2.0% to 3.75% titanium, from 0.7% to 2.0% aluminum, the sum of the aluminum and titanium being within the range between 3.0% and 5.75%, less than 1.0% vanadium, from 0.007% to less than 0.03% boron, and from 25.0% to 32.0% iron with incidental impurities and which is characterized :by exhibiting substantially the same level of rupture properties when measured in the longitudinal and in the transverse directions.

4. An article of manufacture for use in highly stressed parts operating at temperatures of up to at least 1500 F. and higher formed from an austenitic solution hardened and precipitation hardening intermediate iron-nickel base alloy consisting essentially of, 0.10% maximum carbon, 0.5% maximum manganese, 0.5 maximum silicon, from 14.0% to 16.0% chromium, from-42.0% to 48.0% nickel, from 3.25% to 4.5 molybdenum together with from 3.0 to 5.0% tungsten, the sum of the molybdenum and tungsten being within the range between 6.25% and 9.0%, from 2.7% to 3.3% titanium, an from 0.7% to 1.3% aluminum, the sum of the aluminum plus titanium being within the range between 3.4% and 4.6%, 0.5 maximum vanadium, from 0.007% to 0.016% boron, and from 25.0% to 32.0% iron with incidental impurities and which is characterized by exhibiting substantially the same level of properties when measured in the longitudinal and in the transverse directions.

References Cited in the file of this patent UNITED STATES PATENTS I 2,519,406 Scott et a1. Aug. 22, 1950 2,860,968 Boegehold et al. Nov. 18, 1958 2,873,187 Dyrkacz et al Feb. 10, 1959 FOREIGN PATENTS 1,150,704 France Aug. 19, 1957 

1. A PRECIPITATION HARDENING WROUGHT INTERMEDIATE IRONNICKEL BASE ALLOY SUITABLE FOR USE AT ELEVATED TEMPERATURES OF UP TO AT LEAST 1500*F. AND HIGHER, CONSISTING OF, LESS TAHN 0.12% CARBON, LESS THAN 1.5% MANGANESE, LESS THAN 1.5% SILICON, FROM 13.0% TO 16.0% CHROMIUM, FROM 42.0% TO 48.0% NICKEL, FROM 3.0% TO 5.0 MOLYBDENUM AND FROM 2.0% TO 5.0% TUNGSTEN, THE SUM OF THE MOLYBDENUM AND TUNGSTEN BEING WITHIN THE RANGE BETWEEN 6.25% AND 9.0%, FROM 2.0% TO 3.75% TITANIUM AND FROM 0.7% TO 2.0% ALUMINUM, THE SUM OF THE ALUMINUM AND TITANIUM BEING WITHIN THE RANGE BETWEEN 3.0% AND 5.75%, LESS THAN 1.0% VANADIUM, FROM 0.007% TO LESS THAN 0.03% BORON, AND FROM 25.0% TO 32.0% IRON WITH INCIDENTAL IMPURITIES, SAID ALLOY BEING CHARACTERIZED BY EXHIBITING SUBSTANTIALLY THE SAME LEVEL OF RUPTURE PROPERTIES WHEN MEASURED IN THE LONGITUDINAL AND IN THE TRANSVERSE DIRECTIONS. 